The interest toward special sugars and sugar alcohols has recently increased due to the interesting functional properties of some special sugars and sugar alcohols, such as xylitol. Because of the non-cariogenic properties and the reduced calorie value of sugar alcohols compared to conventional carbohydrate sweeteners, sugar alcohols are suitable for the production of non-cariogenic and reduced-calorie (light) products and products with a defined nutritional purpose (e.g. products for diabetics). The sugars of interest may be monosaccharides, disaccharides or higher saccharides. The best-known special sugar alcohols (alditols) are xylitol and sorbitol, but even other sugar alcohols, such as lactitol, maltitol and mannitol have recently gained more interest and more market.
Sugar alcohols may be commercially produced from the corresponding sugars through catalytic hydrogenation. Alditol sugars, such as lactitol, mannitol, sorbitol, xylitol, maltitol etc. are thus produced by the catalytic hydrogenation of the corresponding aldosess, i.e. lactose, mannose, glucose, xylose and maltose. Sponge type nickel catalysts, such as Raney nickel catalysts represent one example of the catalysts generally used in the hydrogenation of sugars, such as in the hydrogenation of aldoses and ketoses to corresponding alditols.
One of the problems encountered in the catalytic hydrogenation of aldose sugars is the deactivation and unstability of the hydrogenation catalyst, for example due to the formation of harmful by-products, such as epimers, hydrolysis products and their reduction products. Aldonic acids, such as lactobionic acid and xylonic acid, represent one example of the harmful by-products formed in the hydrogenation of aldoses to alditols. In the hydrogenation of glucose, it has been found that gluconic acid is typically formed as a by-product. It has also been found that gluconic acid has a tendency to adhere to the catalyst surface thus occupying the active sites of the hydrogenation catalyst and deactivating the catalyst. The deactivation and unstability of the catalyst also lead into problems in the recovery and regeneration of the catalyst. These problems are even more severe specially with recycled catalysts. Recovery of the catalyst by filtration may be difficult.
In the hydrogenation of lactose, for example too high temperatures may lead to the hydrolysis of lactose, whereby galactitol and sorbitol are formed as by-products. As a still further by-product, lactulose is formed due to the isomerisation of lactose, especially at a too high pH. Lactulose is further hydrogenated to lactulitol and lactitol.
R. Albert et al. describe the catalytic production of sugar alcohols and their use in Chem-Ing.-Tech 52 (1980), Nr. 7, p. 582-587. The article surveys numerous applications of the principal sugar alcohols sorbitol and xylitol. It is recited that the industrial production of sugar alcohols takes place almost exclusively by catalytic hydrogenation of the corresponding sugars. Sorbitol is thus manufactured by hydrogenation of D-glucose, xylitol by hydrogenation of xylose and mannitol and sorbitol by hydrogenation of invert sugar or fructose (over sponge nickel catalyst fructose is hydrogenated to both mannitol and sorbitol in approximately equal amounts). Raney nickel is recited to be an especially preferred catalyst. A temperature in the range of 120 to 150° C. is recited as a suitable hydrogenation temperature for the hydrogenation of glucose to sorbitol. The formation of D-gluconic acid as a by-product through the Cannizzaro reaction is also disclosed.
P. Linko et al. describe the preparation, characteristics and potential applications of lactitol in Carbohydrate Sweeteners in Foods and Nutrition, ed. by Pekka Koivistoinen and Lea Hyvönen, University of Helsinki, Academic Press 1980, p. 243-257. The reference discloses reduction with sodium borohydride and catalytic hydrogenation as potential methods for the production of lactitol. Temperatures in the ranges of 100 to 200° C. depending on the hydrogenation pressure are described for the hydrogenation of lactose to lactitol. It is also recited that severe reaction conditions, for example high temperatures (130° C.) lead into the formation of by-products, such as lactulose (by epimerization) and galactose and glucose (by hydrolysis), which in turn are partially hydrogenated to corresponding sugar alcohols lactulitol, galactitol and sorbitol. It is also recited that lactitol is partially decomposed under severe reaction conditions.
Blaise J. Arena (UOP) has studied the deactivation of ruthenium catalysts in continuous hydrogenation of glucose in Applied Catalysis A, General, 87 (1992), p. 219-229, Elsevier Science Publishers B.V., Amsterdam. In this study, several Ru/Al2O3 glucose hydrogenation catalysts were tested in continuous operation. After use, spent catalysts were examined to determine what changes had accompanied deactivation. One of these changes was found to be the build-up of gluconic acid on the catalyst and poisoning of the catalyst by gluconic acid during use.
B. W. Hoffer et. al discuss the role of the active phase of Raney-type Ni catalysts and their Ni—Al alloy precursors in the selective hydrogenation of D-glucose to D-sorbitol in Applied Catalysis A: General 253 (2003), p. 437-452. It is recited that Raney-type Ni catalysts lose Ni and Al at the applied reaction conditions. Furthermore, it is recited that the major cause of deactivation of Raney-type Ni catalysts is the presence of D-gluconic acid formed during the reaction, because D-gluconic acid blocks the Ni sites of the catalyst.
M. Besson et al. discuss the deactivation of metal catalysts in liquid phase organic reactions in Catalysis Today 81 (2003), p. 547-559. The hydrogenation of glucose to sorbitol on Ru/C catalysts has been studied. It is recited that the main by-products are gluconic acid formed by the Cannizzaro reaction and mannitol formed by sorbitol epimerization. Furthermore, it was generally concluded that the main causes of catalyst deactivation are metal and support leaching, deposition of inactive metal layers or polymeric species, and poisoning by strongly adsorbed species.
Ming Hu et al. have presented HPLC and NMR study of the reduction of sweet whey permeate in J. Agric. Food Chem. 1996, 44, p. 3757-3762. It is recited that with a reaction time of 4 hours, an initial hydrogen pressure of 1500 psi, Raney Ni catalyst, and a temperature of 120° C., sweet whey permeate gives lactitol (85.2%), lactulitol (1.7%) and sorbitol and dulcitol (0.8%).
H. E. J. Hendriks et al. have studied the effect of bismuth on the selective oxidation of lactose to sodium lactobionate on supported palladium catalysts at temperatures up to 333 K (60° C.) in Carbohydrate Research, 204 (1990), p. 121-129. It is recited that fifteen batches of lactose were oxidized with the same charge of catalyst without significant loss in initial activity or selectivity. It is also recited that other aldoses such as maltose, glucose and galactose could be oxidized analogously with similar activities.
G. de Wit has studied the catalytic dehydrogenation of reducing aldose sugars to aldonic acids in alkaline solution in Carbohydrate Research 91 (1981), p. 125-138. The dehydrogenation was carried out in alkaline medium under the catalytic action of platinum or rhodium.
U.S. Pat. No. 4,433,184, HRI Inc. (published Feb. 21, 1984) discloses a process for producing a high-purity alditol, such as sorbitol by catalytic conversion of the corresponding monosaccharide sugar to an alditol, whereby the pH of the reaction liquid in the hydrogenation zone is controlled to a value between 4.5 and 7 by adding an alkali solution, such as sodium hydroxide to the hydrogenation feed. The hydrogenation temperature was 130 to 180° C. and the hydrogenation pressure was 500 to 2000 psig. High-activity nickel on an inert support was used as the catalyst. It was found that the alkali addition substantially prevented the undesirable formation of acids such as gluconic acid in the hydrogenation zone, thus preventing the acid leaching of active metals from the catalyst and thereby maintaining high catalyst activity and a long catalyst lifetime.
U.S. Pat. No. 4,510,339, UOP Inc., Blaise J. Arena (published Apr. 9, 1985) discloses an improved method of hydrogenating carbohydrates to corresponding polyols using a group VIII metal as the hydrogenation catalyst, whereby hydrogenation conditions are adjusted so that the content of dissolved oxygen in the carbohydrate feed immediately prior to contacting with the catalyst is less than 0.5 ppm. The hydrogenation catalyst may be for example a ruthenium catalyst. The carbohydrate may be a monosaccharide, such as glucose or mannose, for example. It was found that the use of feeds which had a low dissolved oxygen content resulted in substantially increased lifetime of the hydrogenation catalysts through the reduction of gluconic acid in the spent catalyst. A temperature of 120° C. and a pressure of 2300 psig were used in the hydrogenation of glucose.
U.S. Pat. No. 5,162,517, Bayer Aktiengesellshaft (published Nov. 10, 1992) discloses a process for the preparation of an epimer-free sugar alcohol selected form the group consisting of xylitol, sorbitol (D-glucitol), 4-O-β-D-galactopyranosyl-D-glucitol (lactitol) and 4-O-α-D-glucopyranosyl-D-sorbitol (maltitol) by continuous catalytic hydrogenation of the corresponding sugar selected from D-xylose, α-D-glucose, 4-O-β-D-galactopyranosyl-α-D-glucopyranose (lactose) or 4-O-α-D-glucopyranosyl-α-D-glucopyranose (maltose). The hydrogenation is carried out at a hydrogenation pressure of 150 to 500 bar and at a temperature of 60 to 125° C. by a fixed bed process using hydrogenation catalysts in the form of carrier-free moldings. The hydrogenation catalyst comprises one or more elements of the iron group of the Periodic Table of Elements (iron, cobalt and/or nickel). Regarding the hydrogenation temperature of 60 to 125° C., it is recited that lower temperatures would not achieve a substantially quantitative conversion of the sugar, whereas higher temperatures lead to uncontrollable side reactions such as caramelization, ether splitting or destructive hydrogenation, these possibly resulting in discoloration and the formation of harmful by-products.
U.S. Pat. No. 6,124,443, Bayer Aktiengesellschaft (published Sep. 26, 2000) discloses a process for the preparation of the same sugar alcohols as above by continuous catalytic hydrogenation of the corresponding sugars at a hydrogen pressure of 100 to 400 bar and at a temperature of 20 to 70° C. The hydrogenation catalyst comprises one or more elements of the iron group of the periodic table alloyed with elements of subgroup IV and/or subgroup V. Regarding the hydrogenation temperature of 20 to 70° C., it is recited that the hydrogenation temperature should be as low as possible to decrease the energy costs.
U.S. Pat. No. 6,570,043 B2, Battelle Memorial Institute (published on Sep. 19, 2002) discloses a process of converting a sugar to a sugar alcohol by catalytic hydrogenation at a temperature less than 120° C., preferably at a temperature of 90 to 120° C. and at a pressure of 100 to 3000 pounds per square inch gauge hydrogen gas overpressure. The catalyst comprises ruthenium on a titania support. The starting sugar may be glucose or lactose, for example. It is recited that high conversion and good product selectivity are obtained by the use of a low processing temperature.
L. Fabre et al. describe the catalytic hydrogenation of arabinonic acid and arabinonolactones in Catalysis Communications (2001), 2(8), p. 249-253. Aqueous solutions of arabinonic acid in equilibrium with the corresponding γ-lactone and δ-lactone were hydrogenated to arabitol in a batch reactor in the presence of a ruthenium catalyst supported on active carbon. Introduction of small amounts of sodium anthraquinone-2-sulfonate decreased the reaction rate, but increased the selectivity from 93.6% to 97.9%. It is also recited that the presence of these molecules, which remain adsorbed on the catalyst surface even after successive recycling of the catalyst, decreased markedly the rate of dehydroxylation reactions leading to unwanted deoxy-products.
U.S. Pat. No. 6,486,366 B1, Degussa A G (published Nov. 26, 2002) discloses a method for producing alcohols by reacting a carbonyl compound in a catalytic hydrogenation reaction with hydrogen or hydrogen-containing gases in the presence of a Raney nickel catalyst, which is in the form of hollow bodies. Particularly preferred embodiments of the method relate to the production of sorbitol from dextrose, a mixture of sorbitol and mannitol from fructose, xylitol from xylose, maltitol from maltose, isomaltitol from isomaltose, dulcitol from galactose and lactitol from lactose. It is recited that pH can be adjusted with acid compounds like sugar acids, sorbic acid or citric acid. Furthermore, it is recited that in a continuous process it is also possible to conduct the hydrogenation in two or more steps. In this two-step process, the hydrogenation can be carried out in a first step at a temperature in the range between 60 and 90° C. and can be completed in a second step at a temperature from 90 to 140° C.
Regarding the hydrogenation of lactose to lactitol, the prior art does not disclose the formation of lactobionic acid as a by-product in the lactose hydrogenation process. Consequently, the prior art does not disclose or suggest any means to overcome the problems relating to the formation of aldonic acids, such as lactobionic acid and xylonic acid in the hydrogenation of the corresponding aldose toalditol e.g. due to the high hydrogenation temperature.